wiring speakers in series or parallel? - Model Railroader Magazine

If you add two 8 ohm in series, you are creating a 16 ohm speaker set from the decoder's viewpoint.  If two in parallel, you are presenting 4 ohms to the decoder.  

I'm not in tune with the TCS WowSound diesel particulars, but see in the attached link that one EMD decoder is "optimized" for 8 ohms.  

If you want two speakers (more air movement capability) you would then be steered to using two 4-ohm speakers wired in series.  But you may want to ask TCS what they recommend basis their experience.  It's possible they will say that 4 ohms presented will overload the decoder (and ruin it).  Not sure what they would say about 16 ohms presented.  Alternately, you could technically wire 4, 8 ohm speakers to present 8 ohms to the decoder, by wiring 2 pairs in series (these are then 16 ohms per pair) and wiring the pairs is parallel, which makes the combo 8 ohms.  But fitting four with enclosures into the body would be tough, unless sugarcube types. The following has some good diagrams on multi-speaker wiring:

By analogy, I've used some LokSound Select decoders that are optimal at 4 ohms but 8-16 ohms can also be presented.  These come with 4 ohm speakers.  In a couple cases I installed multi (4) sugarcube speakers that presented the 8 ohms and they worked fine, and I still had to turn down the master volume from max.  You might also pay attention to wattage, as the WOW decoder page talks about 1+ watt speakers, so I presume it's audio output is around 1 watt nominal.  The LokSounds Selects are provided with 2 watt speakers but I'm unsure what the decoder max output actually can be. 

Do also address the speaker(s) having an enclosure.  This can be a box that comes with it, one you make from styrene, or mounting the speaker so it uses an enclosed portion of the body (e.g., sealed portion of roof), boxed in by styrene ends or sides to complete a box.

I had some fun installing LokSound decoder into Proto 2000 SD50 and SD60.  Not sure how the Kato and 90 series would make it easier or more difficult.  In my case, I was able to place the decoder above the motor.  At the long hood end, I milled the weight down so a styrene (not big) enclosure with 2 13x18mm sugarcube speakers could fit, and added 2 11x15mm sugarcube speakers in the cab (with styrene box sides).  A rather tight fit.  IMHO, two sugarcubes will outperform at least some 18mm wide ovals that fit the narrow hood diesels, four sugarcubes is better than two, but the best I've done is two 1.1" round high bass speakers that fit nicely in a E-series diesel but are too wide for the 13mm wide long hoods.  

Maybe someone can comment better on your application; i.e., whether 16 ohms is ok for the WOW Sound, or what 4-ohms speakers to acquire if you want two but not at 16 ohms.